class Chartd
  module Encoder
    B62 = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'.freeze

    # encode encodes a dataset to a format that chartd.co understands.
    # It optionally takes min and max values that change the resulting chart.
    def self.encode(dataset = [], min: nil, max: nil)
      return '' if dataset.empty?

      # either use custom min & max values or take them from the dataset
      min ||= dataset.min
      max ||= dataset.max

      range = dim(max, min)

      # if the range of the data is
      return B62[0] * dataset.count if range == 0

      enclen = B62.length - 1
      encoded = dataset.map do |v|
        index = (enclen * (v - min) / range).to_i

        # TODO: see if else case is even possible
        if index >= 0 && index < B62.length
          B62[index]
        else
          B62[0]
        end
      end

      encoded.join
    end

    # dim returns the maximum of x-y or 0.
    # It is used to calculate the range of the dataset.
    def self.dim(x, y)
      # TODO: maybe raise an exception if max < min (x < y)
      return 0 if x < y

      x - y
    end
    private_class_method :dim
  end
end